Fiber lasers have the significant advantages of high efficiency, easy thermal management and inherent robustness. With recent advances in high quality, double-clad fibers, fiber lasers have been demonstrated to achieve power conversion efficiencies of over 90%, and power levels approaching 1 kW in continuous wave (cw) operation. For pulsed operation, however, fiber lasers often need free-space devices (such as acousto-optical or electro-optical switches) to initiate and maintain stable operation. However, the use of such free-space devices generally diminishes the inherent advantages of the confined structured optical fiber, and typically leads to problems in system complexity and reliability.
A gain-switched fiber laser generally has the following advantages:
(i) All-fiber, in-line pulse generation—no free-space optics are required.
(ii) Flexible repetition rate—gain-switched fiber lasers can produce a stable pulse train at any modulation rate up to the relaxation oscillation frequency, which is typically in the range of MHz.
(iii) Wide range of pulse widths are possible, from ns to μs—the pulse width of gain-switched lasers depends on gain and the “cavity lifetime”, which can be varied over a wide range in fiber lasers.
Gain-switching is a pulse generation method which generates pulsed output by varying the gain of a laser cavity. In fiber lasers, the gain-switching is achieved within the gain fiber, which typically comprises an optical fiber doped with a rare-earth element which serve as active ions, which makes it possible to achieve a potentially in-line pulse generation.
However, the challenge of generating a stable pulse chain in gain-switched fiber lasers lies in the control of the relaxation oscillation, which typically results in chaotic pulsation.
In principle, laser dynamics can be described by using rate equations for population inversion, N(t), and the cavity photon number P(t):dN(t)/dt=R(t)−N(t)/τE−K·N(t)·P(t)  (1)dP(t)/dt=K·N(t)−P(t)−P(t)/τP+K·N(t)·PASE  (2)where R(t) is the effective pump rate for population buildup of N(t), K is the coupling coefficient, PASE are the spontaneous emission photons coupled into the laser mode, τE is the excitation lifetime and τC is the cavity (photon) lifetime. The cavity lifetime τC=TC/δC is determined by the cavity roundtrip time TC and cavity loss δC. For fiber lasers which typically have a large output coupling, the cavity lifetime is essentially the same as the cavity roundtrip time, namely, τC≈TC=L/2C, with L being the effective cavity length and C the speed of light.
Under continuous wave (cw) operation, the laser output has a relatively stable amplitude, except for the short time interval when the pump source is first turned on. For this short time interval, the excitation population builds up initially according to Eqs. 1 and 2, to a level near the lasing threshold (Nth=1/KτC) and the photon numbers begin to grow exponentially. The growth rate of the photons is essentially given by (K·N(t)−1/τC), which grows larger as the N(t) increases with continuous pumping. As the growth rate of the photons exceeds the pumping rate, the excitation population eventually decreases below the lasing threshold and the lasing stops. This photon buildup and lasing action is repeated until the laser operation reaches a steady state with constant laser output (dP/dt=0), when the excitation population supplied by the pump is equal to that consumed by the photons. The oscillation buildup time (τB), the period (ωRe) and amplitude damping rate (γRe) of the relaxation oscillation can be approximated as:τB=CS·τC/(γ−1)  (3)ωRe=[(γ−1)/τEτC]0.5  (4)γRe=γ2τE  (5)where γ=RτE/Nth is the inversion ratio, and CS≈20-30 is a constant for a given laser representing the log ratio of the steady-state light intensity and that at initial noise level. The buildup time, relaxation oscillation period and damping rate all depend on the pumping rate, the cavity lifetime and the lifetime of the excited state.
Under gain-switched operation, using a modulated pump with a sufficiently large amplitude contrast and time period, the multi-peak relaxation oscillation pattern may be repeated following each pump pulse. The laser buildup, oscillation period and amplitude damping are, in principle, similar as that in Eqs. 3-5, with the population inversion ratio adjusted according to the effective pump rate R(t). The effective pumping rate in a typical laser system, however, is a function of the modulation pattern of the pump laser and, more importantly, a function of the relaxation dynamics from the pump absorption energy level (EP=hc/λP) to the meta-stable energy level ES=hc/λS where the excitation population N(t) is accumulated. The relaxation oscillation pattern may vary significantly depending on the energy levels and time scales involved in the excitation population buildup process.
Relaxation oscillation is a characteristic of most solid state lasers in which the recovery time of the excited state population is substantially longer than the laser cavity decay time. The phenomenon was observed as early as the first generation of ruby lasers. Because of the long lifetime of the gain medium, relaxation oscillation is particularly severe in fiber lasers. Chaotic pulsation has been commonly observed. For example, in a gain-switched thulium-doped fiber laser pumped by a Ti:Sapphire laser at 790 nm, the output pulse train is somewhat periodic but the pulse width and frequencies are different than those of the pump pulses: the pulse width is ˜0.3 μs and separated by 1 μs for the thulium-doped fiber laser (TDFL) output while the pump pulses are separated by 0.67 μs. Moreover, the laser dynamics in fiber lasers can be particularly complicated when the laser dynamics involve more than the typical 3-level or 4-level laser system. One outstanding example is the excited state absorption in thulium-doped fiber lasers. When the pump wavelength is near 1 μm, the absorption cross-sections of the excited states are on the same order as the ground state absorption. The decay time of the excited state absorption upper energy level is also on the same order as the laser population. Under this condition, the spiking phenomenon in a thulium-doped fiber laser becomes totally chaotic.